1. Field of the Invention
The present invention relates in general to a radiotelephone such as a mobile telephone, and more particularly to a telephone hand-free apparatus which can use a portable telephone (viz., portable handset radiotelephone) as a hand-free telephone when having the portable telephone connected thereto. More specifically, the present invention is concerned with the telephone hand-free apparatuses of a type which exhibits excellent tone quality and excellent power saving.
2. Description of the Prior Art
Nowadays, mobile telephones are available for drivers seated in the motor vehicles. However, the use of a portable telephone (viz., portable handset radiotelephone) by the driver causes he or she to handle the steering wheel with one hand, which of course lowers the driving safety. In order to avoid such one hand handling, that is, in order to allow the driver to communicate with a person at the other end of the communication line without holding the portable telephone, various apparatuses have been proposed and put into practical use, some of which are of a so-called "telephone hand-free apparatus" which can use the portable telephone as a hand-free telephone when having the portable telephone connected thereto.
For ease of description, in the following, the person at the other end of the communication line will be referred to as the other person and a telephone used by the other person will be referred to as the other telephone.
Usually, the telephone hand-free apparatus is mounted in a vehicle cabin near the driver's seat, and comprises generally a case, a microphone mounted to the case for picking up a voice of the driver, a speaker installed in or mounted to the case for transmitting the other person's voice to the driver, a control circuit installed in the case for controlling operation of the microphone and the speaker, and a battery installed in the case for powering the microphone, speaker and control circuit.
However, due to inherent construction, the telephone hand-free apparatuses of the above-mentioned type tends to have the following drawbacks.
First, due to the close positioning between the speaker and the microphone, the telephone hand-free apparatus tends to produce uncomfortable howling when in use. That is, under communication between the driver and the other person, a voice produced by the speaker of the telephone hand-free apparatus is inevitably picked up by the microphone of the same and its voice signal is returned to the same speaker after making a round travel through the other telephone, and a voice thus produced by the speaker from the voice signal is picked up by the same microphone again, resulting in that the voice signal is forced to make repeated travels between the two telephone devices, which causes generation of ultra-high frequency sounds, that is, the uncomfortable howling. One measure for suppressing or at least minimizing such howling is proposed by Japanese Patent First Provisional Publication 5-14476. That is, in that measure, a howling or echo canceling unit is installed in the control circuit for reducing a loop gain of a first loop. However, as is known, such howling canceling unit is very complicated and expensive. Thus, if the unit is practically installed in the telephone hand-free apparatus, the cost of the apparatus becomes very increased.
Second, if the portable telephone is set close to the control circuit of the telephone hand-free apparatus, it often occurs that electromagnetic wave noises inevitably emitted from the portable telephone affect the control circuit and thus lower and deteriorate the tone quality of the telephone communication.
Third, in the hitherto proposed telephone hand-free apparatuses, power saving has been given little thought. Thus, the battery used in the apparatus tends to have a shorter life-time.